The bifidobacteria are obligatory anaerobic, gram-positive bacilli always present in human intestine, and are known as useful bacteria which amount to 90% or more of intestinal microflora particularly in infants. Research on the interaction between enteric bacteria and a human being, a host, from the viewpoint of two aspects, i.e., harmfulness and usefulness is rapidly proceeding. The bifidobacteria which are main bacteria of intestinal microflora are known as bacteria having an inhibitory activity on colonization of harmful bacteria such as pathogenic bacteria in intestine, a suppressive activity on abnormal proliferation thereof in intestine, and a preventive activity on intoxication, intestinal infection and the like caused by harmful bacteria. Particularly in the case of babies whose immunity has not yet been developed, their intestinal microflora is of important significance as a flora which imparts a power of resistance to infectious diseases, and it is considered that proxotion of formation of a healthy intestinal microflora by the bifidobacteria is an effective means for reducing the danger of intestinal infection in babies. Intestinal infections often caused in infants are roughly classified into toxin type diseases caused by toxins produced by pathogenic bacteria, and infection type diseases caused by direct contact of pathogenic bacteria with intestinal tissues or cells and their invasion thereupon. The intestinal bifidofacteria produces acetic acid and lactic acid which inhibit proliferation of these pathogenic bacteria or their toxin production, and has an activity of directly inhibiting proliferation of the pathogenic bacteria. Nakaya et al. have reported that the bifidobacteria have a preventive activity on intracellular infection with Shigella flexneri which has the properties of invading cultured human cells (The Journal of Pediatric Practice Vol.47, No.5 P.723-728 (1984)).
On the other hand, as to the cell wall of the bifidobacteria or its constituents, utilization thereof as antitumor agents or carcinostatic agents has heretofore been known (Japanese Patent Publication No. 42271/81 and Japanese Unexaxined Patent Application Publication Nos. 103194/81, 212122/82 and 118712/84), but the anti-infective activity of these substances has not yet been reported. Further, all of the active materials in these inventions are characterized by containing peptides or proteins.
In order to develop a drug capable of preventing intestinal infection and infection in eyelid or superior trachea mucosa which are caused in infants and aged men, the present inventors noted the inhibitory activity of viable cells of the bifidobacteria on invasion of Shigella flexneri upon cells and searched for the active material of the bifidobacteria. As a result, the present inventors have found that a polysaccharide which is a constituent of the cell wall of the bifidobacteria has an inhibitory activity on invasion of Shigela flexneri upon cells, and have accomplished this invention on the basis of this finding.